Method of treating backfill

ABSTRACT

A method is disclosed for preparing excavated soil to render it suitable as hardened backfill by mixing with the soil a cementitious substance and ice particles, the ice particles being used in lieu of the water normally required to react with the cementitious substance.

This invention is a method for treating excavated soil with acementitious substance and ice particles so as to render the soilsuitable as backfill which will be at least as stable and as compact asadjacent undisturbed soil, or less stable, if desired.

BACKGROUND

Excavated soil is usually required as backfill around a constructionsite, and the stability of the backfill is often required to equal orexceed that of the adjacent undisturbed soil. Thus, the soil of a road,runway or the like usually requires greater strength than the adjacentnatural soil strength. Conventionally, with respect to theserequirements, the stabilization process has been performed as follows.First, the mixture of a cementitious material, such as cement, lime,gypsum, water-glass, or the like, and the excavated soil is backfilledat the excavation site and then the backfill is compacted by rolling,tamping or the like, while solidification of the cementitious materialoccurs.

In this stabilization process the ratio of the cementitious material tothe excavated soil is relatively small, namely less than 10%.Furthermore, in order to maximize the strength of the mixture of soiland cementitious material, it is necessary to use optimum water content.However, there are many grades of soil, some of which are not amenableto homogeneous mixing with a cementitious material and water. Even withsoils relatively amenable to mixing, it has been difficult to mix soilhomogeneously with a small amount of cementitious material and water.

There are two Japanese Patent Publications (B2) Nos. Sho 51-33662 andSho 61-8212 which disclose techniques of treating excavated soil.Publication Sho 51-33662 discloses a method of treatment comprising theblending of soil, cement and sludge in a vessel installed at theconstruction site. As a result of this mixing, the treated soil isalleged to have the desired strength and stability. According to thismethod, the addition of sludge or the like is done simultaneously withthe addition of cement. Publication Sho 61-8212 discloses the method ofinstalling an excavated soil separating unit, a measuring unit and amixing unit on the excavation site. Soil is excavated and drained ofwater content by the separating unit. The resultant residue, or mud, isreinforced with a cement homogeneously mixed and then backfilled intothe excavation by means of a pressure pump.

However, it has been difficult to mix together the excavated soil,cementitious material and water homogeneously by either method. Thestrength of the solidified soil is liable to fluctuate because thephysical qualities of the soil will vary from one construction site toanother. For instance, in soil which contains gravel and rock,separation may occur similar to the phenomenon of separation ofaggregate in concrete. Furthermore, it has also been difficult in a caseof particularly soft cohesive soil to homogeneously disperse a smallamount of water and cementitious material.

Previous to this application, the Applicant invented a method to preparehomogeneous, high quality mortar and concrete in which there is noseparation of the aggregate and excellent workability is obtained byusing small ice particles in lieu of water and then causing the gradualmelting of the ice particles in the stage of mixing the cement andaggregate. (See International Laid-Open patent applications Nos.W086/00884 and W087/00163). In addition, Japanese Patent Publication(B2) No. Sho 53-005694 has disclosed a method of manufacturingconstruction boards, which comprises the steps of mixing a powderyhydraulic binder with crushed ice in a form like snow at a temperaturebelow the melting point of water substantially in a dry state,compressing the resultant substantially dry mixture to a predeterminedthickness and then solidifying the resultant layer at a temperatureabove the melting point of water. Further, this method has suggestedthat a porous board is manufactured by adjusting the melting of ice inthe stage of solidification.

It is therefore an object of the present invention to produce treatedbackfill in which ice particles are used in lieu of part or all of thewater required for adding to the mixture of excavated soil andcementitious material. The mixture having high homogeneity iseffectively produced by making use of characteristics of ice particlesin the solid phase or in the quasi-solid phase resulting from themelting of the surfaces thereof, thereby making it possible to treat theexcavated soil to provide it with the desired physical properties at lowcost.

DESCRIPTION OF THE INVENTION

The invention comprises a method for blending a cementitious materialwith excavated soil by means of a mixer. The quantity of water necessaryto bring the water in the mixture to a predetermined content is added inthe form of small ice particles. In the initial stage of mixing, thesmall ice particles behave as the solid phase or the quasi-solid phaseresulting from the partial melting of the small ice particles to carrysoil or cementitious components on the ice particle surfaces thereof. Amacroscopically homogeneous mixture may be readily obtained throughagitation of the soil, cementitious material and small ice particles fora short period of time. Subsequently, the small ice particles aregradually melted and the resultant water wets the surrounding soil orcementitious substance to cause a transition to a homogeneous mixture.This method of soil treatment is not limited to a method of backfillingand compacting the mixture only after obtaining the homogeneous mixtureresulting from the melting of the ice particles. The mixture may bebackfilled before the ice particles are melted. Thereafter the small iceparticles are gradually melted to cause the transition to a homogeneousmixture. By so doing, operating efficiency may be remarkably improvedsince the mixture may be backfilled in the ground approximately in a drystate, thereby shortening the mixing time.

Since the excavated treated soil may be used to backfill in theexcavation it is not necessary to haul the excavated soil to a dumpsitefor disposal. Of course the treated excavated soil may be transportedfor backfill at some other construction excavation site.

If the excavated soil does not contain sufficient moisture, additionalmake-up water may be mixed with the cementitious material and the smallice particles. However, when the soil is saturated with water, excesswater should be separated by a separating unit and then the small iceparticles added to the dehydrated soil at the time of mixing andkneading with the cementitious material.

Examples of cementious materials which may be used in the invention arePortland cement, gypsum, lime, water-glass or the like. These cementsbind the soil particles to each other to improve the strength of thesoil after solidification of the mixture.

The particle size of ice is 30 millimeters or less. When the earth andatmospheric temperatures are high, ice particles having relatively largediameters may be used. On the contrary, when said temperatures are low,ice particles having relatively small diameters may be used. The amountof ice particles to be incorporated is set by subtracting the naturalwater content contained in the earth from the optimum water content, or,if necessary, by adjusting the amount of water according to the degreeof compaction or the like, for example. After the mixture has beenbackfilled and compacted, proper measures may be taken to prevent thesurface from drying, if necessary.

In another embodiment of the invention, the mixture of cementitiousmaterial, such as Portland cement, ice particles having relatively largediameters and excavated soil having a high percentage of gravel and rockis backfilled to start solidification. At this time, the ice particlesbehave as solids, even if the mixture has a high water-cement ratio andthe fluidity at the time of backfill is low. Therefore, the space to bebackfilled is not compactly filled. With the progress of solidificationof the mixture, the ice particles are gradually melted to form cavities.As a result, a coarse solidified composition having low strength may beobtained from the high water-cement ratio mixture.

For example, when the mixture with relatively large diameter iceparticles is backfilled into a vertical mine shaft having a depth ofseveral hundred meters, the ice particles are gradually melted to formthe solidified composition with cavities resulting from the melting ofice particles. Further, since the ice particles behave as solids, thefluidity of the mixture is low, even if the water-cement ratio isrelatively high, and the interior of the shaft, therefore, is notcompactly backfilled. However, the solidified backfill has sufficientstrength to prevent the shaft from collapsing, and because of its lightcompaction, the backfill is easily re-excavated.

For light compaction, the ice particles having diameters within therange from 10 to 100 millimeters are preferable. Large diameter iceparticles are preferred when the surrounding ambient temperature is highand small diameter ice particles are preferred when the ambienttemperature is low. Coarse compacting may be enhanced by adding to themixture a substance having viscosity-increasing properties, such asmethylcellulose. The methylcellulose may be incorporated with themixture or dissolved in and mixed with the ice particles whereby thecoarse mixture will be lightly compacted when backfilled.

A preferred method of designing a proper mixture of soil, cement, waterand ice particles includes the following considerations and procedures.

Assuming that the excavated soil has an unconfined compressive strengthof 1.0 kg/cm² or less, which must be upgraded to have an unconfinedcompression strength of about 10.0 kg/cm², the following steps arerecommended.

(i) Examine the water content in per cent of dry weight, and the gradingdistribution and consistency of five samples of excavated soil.

(ii) Mix cement into the five samples of soil in the ratios of 2, 4, 6,8 and 10%, respectively, and then obtain the optimum water content inper cent of dry weight and the maximum dry density by compaction andpenetration tests.

(iii) Conduct unconfined compression tests on each sample at the optimumwater content in per cent of dry weight after solidification and thengraph the relation between the amount of cement and the strength.

(iv) Determine from the graph the amount of cement required forunconfined compression test strength of 10.0 kg/cm².

When the natural water content in percent of dry weight is very high,the test is conducted on the sample after the natural water content hasbeen separated. Further, in the experimental kneading with small iceparticles used in lieu of water, the test may be conducted underconditions closely similar to those of the method according to thepresent invention.

Generally, the strength of treated soil mixed at the site is less thanthat mixed in a laboratory by about 20% to 40%. After the correct amountof cement and water content for the mixture has been determined fromtesting soil samples, the following is a preferred procedure forpracticing the invention.

First, the soil is excavated and excess water is separated from thesoil. Next the dehydrated soil is transported to a mixing unit where theproper percentages of cement and ice particles are mixed with the soil.

The size of ice particles is properly selected according to the mass ofsoil to be mixed, the required mixing time and the atmospherictemperature. When the amount of soil is relatively small and thetemperature is relatively low, ice particles having diameters of up toseveral millimeters may be used. Even snow or the like may be used undercertain conditions. When the amount of soil is large and the temperatureis high, it may be necessary to use ice particles up to 30 millimetersin diameter.

After mixing, the mixture is then removed from the mixing unit andtransported to a tank from which the mixture may be pumped back into theexcavation. Upon solidification the mixture will acquire strength due tothe hydraulic reaction of the cement, water and soil.

It will occur to those skilled in the art, upon reading the foregoingdescription of the preferred embodiments of the invention, that certainmodifications may be made to the invention without departing from itsintent or scope. It is intended, therefore, that the invention only beconstrued and limited by the appended claims.

What is claimed is:
 1. The method of preparing soil for back-filling anexcavation and backfilling thereof comprising the steps of:(a) removingexcess water from said soil; (b) homogeneously mixing a cementitiousmaterial with said soil; (c) homogeneously mixing small ice particleswith said soil, the temperature of which is above the freezingtemperature of water; (d) backfilling the homogeneous mixture of soil,cementitious material and small ice particles into an excavation.
 2. Themethod of claim 1, including the step of backfilling said mixture whilethe ice particles are in the solid state phase.
 3. The method of claim1, including the step of backfilling said mixture while the iceparticles are in the quasi-solid state phase.
 4. The method of claim 1,including the step of adding to the mixture a substance havingviscosity-increasing properties.
 5. The method of claim 1, including thestep of adding methylcellulose to the mixture in sufficient amount toincrease the viscosity of the mixture.
 6. The method of claim 1, whereinthe ratio of cementitious material to soil is within the range of 2 to10%.
 7. The method of claim 1, wherein sufficient cementitious materialis mixed with the soil to yield an unconfined compression strength ofsubstantially 10.0 kg/cm².
 8. The method of preparing soil forbackfilling an excavation comprising the steps of:(a) adjusting thewater content of the soil; (b) in a mechanical mixer, at any ambienttemperature above freezing, homogeneously mixing soil, a cementitioussubstance, and solid state ice particles; (c) permitting the iceparticles to melt sufficiently to at least wet and stick the soil andthe cementitious substance to the surfaces of the ice particles; (d)removing the mixture of soil, cementitious substance, and ice particlesfrom the mechanical mixer; (e) transporting said mixture to anexcavation; and (f) backfilling said mixture into said excavation. 9.The method of claim 8, wherein the step of adjusting the water contentof the soil comprises removing excess water from the soil.
 10. Themethod of claim 8, including the step of mechanically compacting saidmixture following backfilling into said excavation.
 11. The method ofclaim 8, including the step of backfilling said mixture while said iceparticles are in the solid phase.
 12. The method of claim 8, includingthe step of backfilling said mixture while said ice particles are in thesemi-solid phase.
 13. The method of claim 8, including the step ofbackfilling said mixture after said ice particles have been fullymelted.
 14. The method of claim 8, including the steps to produce alow-strength composition for coarse compaction comprising the stepsof:(a) selecting a gravel-containing soil; (b) selecting ice particleswithin a range of substantially 10 to 100 millimeters; (c) backfillingthe said mixture before substantial melting of ice particles occurs; (d)lightly compacting the said mixture; and (e) permitting the said iceparticles to melt to form cavities in the compaction.
 15. The method ofclaim 14, including the step of adding a viscosity-changing substance tosaid mixture.
 16. The method of claim 14, including the step of addingmethylcellulose to the said mixture.
 17. The method of claim 14,including the step of backfilling said mixture while said ice particlesare in the solid phase.
 18. The method of claim 14, including the stepof backfilling said mixture while said ice particles are in thesemi-solid phase.